Introduction to the NAVSTAR Global

Positioning System (GPS)

Introduction to the NAVSTAR Global

Positioning System (GPS)

Slide 2

Agenda

GPS Lineage What is GPS How Does It Work Errors and Accuracy's in the GPS

system Future Initiatives

GPS Lineage What is GPS How Does It Work Errors and Accuracy's in the GPS

system Future Initiatives

Slide 3

GPS Lineage

Phase 1: 1973-1979 CONCEPT VALIDATION

1978- First Launch of Block 1 SV Phase 2: 1979-1985

FULL DEVELOPMENT AND TESTS Phase 3: 1985-Present

PRODUCTION AND DEPLOYMENT1993-IOC 1995-FOC

Slide 4

What is GPS

Space Segment Control Segment User Segment

Space Segment Control Segment User Segment

The Global Positioning System (GPS) is a Constellation of Earth-Orbiting Satellites Maintained by the United States Government for the Purpose of Defining Geographic Positions On and Above the Surface of the Earth. It consists of Three Segments:

Slide 5

Space Segment Description

Very high orbit 20,200 km

– 1 revolution in approximately 12 hrs

For accuracy Survivability Coverage

Very high orbit 20,200 km

– 1 revolution in approximately 12 hrs

For accuracy Survivability Coverage

24+ satellites 6 planes with 55°

Inclination Each plane has 4 or

5 satellites Broadcasting position

and time information on 2 frequencies

Constellation has Spares

24+ satellites 6 planes with 55°

Inclination Each plane has 4 or

5 satellites Broadcasting position

and time information on 2 frequencies

Constellation has Spares

Slide 6

Control Segment

Master Control Station

Monitor Station

Ground Antenna

ColoradoSprings

Hawaii AscensionIslands

DiegoGarcia

Kwajalein

Monitor and Control

Slide 7

Control Segment

(5) Monitor Stations

• Correct Orbitand clockerrors• Create new navigation message

• Observeephemerisand clock

Falcon AFBUpload Station

Slide 8

User Segment

Over $19 Billion invested by DoD Dual Use System Since 1985

(civil & military) Civilian community was quick to take

advantage of the system Hundreds of receivers on the market 3 billion in sales, double in 2 years 95% of current users

DoD/DoT Executive Board sets GPS policy

PLGR

Slide 9

Common Uses for GPSMilitary Specific:

Navigation Surveying Target acquisition and destruction Missile Guidance Systems

Joint Direct Attack Munition (JDAM) Tomahawk III Joint Stand Off Weapon (JSOW)

Data Collection Integration with INS for High dynamic

environment Search & Rescue Ops

Slide 10

How the system works

Space Segment24+ Satellites

GPS Control Colorado Springs

The Current Ephemeris is Transmitted to UsersMonitor

Stations• Diego Garcia• Ascension Island• Kwajalein • Hawaii• Colorado Springs

End User

Slide 11

Distance Measuring

The whole system revolves around time!!!

Distance = Rate x Time

• Rate = 186,000 miles per second (Speed of Light)

• Time = time it takes signal to travel from the SV to GPS

receiver

Slide 12

Triangulation

Satellite 1 Satellite 2

Satellite 3 Satellite 4

Slide 13

Distance Measuring

Transmission Time

Receiver

Time delay

The Carrier... combined with

The PRN code... produces the

Modulated carrier signal which is transmitted... demodulated...

Producing the same code at the user, but delayed...

Satellite

Slide 14

Signal StructureL1 Carrier Wave 1575.42MHz

C/A Code 1.023 MHz

Navigation Message50Hz

Precise Code 10.23 MHz

Slide 15

Signal StructureL2 Carrier Wave 1227.6MHz

Navigation Message50 Hz

Precise Code10.23 MHz

Slide 16

Measuring Travel TimeSV Clocks

2 Cesium & 2 Rubidium in each SV

$100,000-$500,000 eachReceiver Clocks

Clocks similar to quartz watch Always an error between satellite and receiver clocks ( t) Require 4 satellites to solve for x, y, z, and t

Slide 17

Measuring Travel Time

4

Time Adjustment

X

1 2

3

XX

(wrong time)9 nano seconds(wrong time)

YY ZZ

8 nano seconds (wrong time)

7 nano seconds

Slide 18

3 vs 4 Satellites

Slide 19

Satellite Locations

Cartesian Coordinate System Three dimensional right coordinate system with an origin

at the center of the earth and the X axis oriented at at the Prime Meridian and the Z at the North Pole

X Axis Coordinate Distance in meters from the the prime meridian at the origin; positive from 90º E Long to 90º W Long

Y Axis Coordinate Distance in metersfrom 90º E longitude at the origin; positive in the eastern hemisphere and negative in the western

Z Axis Coordinate Distance in meters from the plane of the equator; positive in the northern Hemispherenegative in the southern

Z

Prime Meridian

X0º Long

Y90°E

(X,Y,Z)

Slide 20

Common Problems - Errors

Direct

Sig

nal

Ref

lect

ed S

igna

l

GPSAntenna

Reflected Signal

Hard Surface

Satellite

Implemented on Block II satellites, but turned off 2 May 2000 for the foreseeable future: Requires military to develop Direct Y Code

receivers and local jamming capability

Introduces deliberate errors into satellite ephemeris (SV location) and clock parameters on the C\A code

Degrades horizontal positional accuracy to 100m 2DRMS (95% of the time.)

Selective Availability (S/A)

Anti-Spoofing (A-S)

P-Code+W-Key

Y-Code

• Protects military receivers from receiving a “fake” P-Code • P-Code modulation on both L1 and L2

• No plans to phase out• Continuously on since January 31, 1994

Resistance to Jamming

• Low power signal is vulnerable to jamming– Intentional or unintentional jamming– Theater wide jamming– Local area jamming

• The P-Code is phase modulated to provide better resistance to jamming• DoD working on electronic warfare enhancements to deny disruption and spoofing.

– Direct Y-Code Receivers– Theater jamming capability

Slide 24

Common Problems - Errors

Satellite clock (S/A) Ephemeris/orbit (S/A) Atmospheric delays

Ionosphere Troposphere

Receiver computation & noise

Satellite clock (S/A) Ephemeris/orbit (S/A) Atmospheric delays

Ionosphere Troposphere

Receiver computation & noise

Pseudo-Ranging Errors

Slide 25

Common Problems - Errors

Errors Caused By GPS Multipath Reflections Use Ground Plane On Antenna Move Away From Reflective Surfaces

Influences on the GPS Signal Radar Microwave ILS or Radio NDB Equipment ATC Radio Traffic

Misidentification of Thresholds and Other Features

Errors Caused By GPS Multipath Reflections Use Ground Plane On Antenna Move Away From Reflective Surfaces

Influences on the GPS Signal Radar Microwave ILS or Radio NDB Equipment ATC Radio Traffic

Misidentification of Thresholds and Other Features

Slide 26

GPS Multipath ErrorsEffects of Multipath on the GPS Signal

• Avoid Reflective Surfaces• Use A Ground Plane Antenna • Use Multipath Rejection Receiver

Direct S

ignal

Ref

lect

ed S

igna

l

GPSAntenna

Reflected Signal

Hard Surface

Satellite

Slide 27

Dilution Of Precision (DOP)A Measure of The Geometry Of The Visible GPS Constellation

Good DOP

Poor DOP

Slide 28

Dilution Of Precision (3)

PDOP = Position Dilution Of Precision (Most Commonly Used)

VDOP = Vertical Dilution Of Precision GDOP = Geometric Dilution Of

Precision HDOP = Horizontal Dilution Of

Precision TDOP = Time Dilution Of Precision

PDOP = Position Dilution Of Precision (Most Commonly Used)

VDOP = Vertical Dilution Of Precision GDOP = Geometric Dilution Of

Precision HDOP = Horizontal Dilution Of

Precision TDOP = Time Dilution Of Precision

QUALITY DOP

Very Good 1-3Good 4-5Fair 6Suspect >6

Mission Planning

Is Crit

ical to O

btain

Good DOP

Mission Planning

Is Crit

ical to O

btain

Good DOP

System Accuracy

Available to all users Accuracy was degraded by Selective

Availability until 2 May 2000 Horizontal Accuracy: 100 meters 2 DRMS (40

meters CEP)

Now has roughly the same accuracy as PPS

Used by military receivers before Y-code lock is established

Standard Positioning Service (SPS)

Slide 30

Scatter plot of horizontal accuracy 2 May 2000

System Accuracy

Only available to authorized DoD users Decryption device and crypto key are

required to decode A-S and remove SA GUV Key (1 year) CVW Key (1 week)

Accurate to 21m 2DRMS (8 m CEP) 95% of the time, a receiver's computed

horizontal position will be within 21 meters of its actual location

Precise Positioning Service (PPS)

GPS Accuracy - PPS

PPS CEP/50 % DRMS 2DRMS/95%

Position

Horizontal 8 m 10.5 m 21 m

Vertical 9 m 14 m 28 m

Spherical 16 m 18 m 36 m

VelocityAny Axis 0.07 m/sec 0.1 m/sec 0.2 m/sec

TimeGPS 17 nsec 26 nsec 52 nsec

UTC 68 nsec 100 nsec 200 nsec

Specifications and Derived Values

Error and Map Problems

XMap coordinatedetermined byterrain association

50 m Map Error

X GPS coordinateplotted on map

21m GPS Error

Slide 34

Differential GPS

Coverage:Coverage: Local Area (Coast Guard) Wide Area (INMARSAT)

Methods:Methods: Real-Time

(navigation/mapping) Post Processing (survey)

Types of Differential Coverage

DGPS Positioning

DGPS Navigation

Initial Operational Capability on 30 Jan 96

Provides pseudo-range corrections over existing radio beacons

Corrections to NAD-83 (WGS-84) Observed accuracy 1 to 3 meters out to

150 nautical miles from base station Station sites available on the internet

(WWW.NAVCEN.USCG.MIL)

Coast Guard Differential GPS System

DGPS Positioning

Wide Area Differential GPS

Reference receivers

GPS signals

Field receiver

Error correctionmessage

Real-time Corrections to Remove S/A etc.

Future DevelopmentsPlanned Replenishments - Block IIR

• Some IIR improvements over Block II/IIA SVs:– More power/better batteries (Life EST 7.8 years)– More fuel– Two Atomic clocks on at all times– Re-programmable CPU, more autonomous– Cross Link Ranging - 180 day autonomy with no degradation

• 21 SVs purchased from Lockheed Martin at $30M each • Launches began Jan 97

Future Developments

Planned Sustainment - Block IIF

• Boeing awarded contract for production of 33 Block IIF SVs• Improvements over IIR

– Larger Payload (more fuel, power, etc)– 10 year life span– DoT option to add L??? & L??? Frequencies – Unique ground control (more autonomous)

Summary

History GPS Applications Three Segments of GPS 5 Principles of GPS Operations System Accuracy Other Satellite Navigation Systems Future Developments